Optical fiber has become increasingly important in many applications involving the transmission of light. When a pulse of light is transmitted through an optical fiber, the energy follows a number of paths which cross the fiber axis at different angles. A group of paths which cross the axis at the same angle is known as a mode. The fundamental mode, also known as the LP01 mode, is the mode in which light passes substantially along the fiber axis. Modes other than the LP01, mode, are known as high order modes. Fibers which have been designed to support only one mode with minimal loss, the LP01 mode, are known as single mode fibers. High order modes exhibit characteristics which may be significantly different than the characteristics of the fundamental mode. There exists both even and odd high order modes. Even high order modes exhibit circular symmetry, and are thus ideally suited to circular waveguides such as optical fibers.
A multi-mode fiber is a fiber whose design supports multiple modes, and typically supports over 100 modes. A few-mode fiber is a fiber designed to support only a very limited number of modes. For the purpose of this patent, we will define a few mode fiber as a fiber supporting no more than 20 modes at the operating wavelength. Few mode fibers designed to have specific characteristics in a mode other than the fundamental mode are also known as high order mode (HOM) fibers. Fibers may carry different numbers of modes at different wavelengths, however in telecommunications the typical wavelengths are near 1310 nm and 1550 nm.
As light traverses the optical fiber, different group of wavelengths travel at different speeds, which leads to chromatic dispersion. This limits the bit rate at which information can be carried through an optical fiber. The effect of chromatic dispersion on the optical signal becomes more critical as the bit rate increases. Chromatic dispersion in an optical fiber is the sum of material dispersion and the waveguide dispersion and is defined as the differential of the group velocity in relation to the wavelength and is expressed in units of picosecond/nanometer (ps/nm). Optical fibers are often characterized by their dispersion per unit length of 1 kilometer, which is expressed in units of picosecond/nanometer/kilometer (ps/nm/km). For standard single mode fiber (SMF), dispersion at 1550 nm is typically on the order of 17 ps/nm/km.
The dispersion experienced by each wavelength of light is also different, and is primarily controlled by a combination of the material dispersion, and the dispersion created by the actual profile of the waveguide, known as waveguide dispersion. The differential of the dispersion in relation to wavelength is known as the slope, or second order dispersion, and is expressed in units of ps/nm2. Optical fibers may be further characterized by their slope per unit length of 1 kilometer, which is expressed in units of picosecond/nanometer2/kilometer (ps/nm2/km).
At high bit rates, compensating for the slope is important so as to avoid xe2x80x9cwalk offxe2x80x9d, which occurs when one wavelength in the band is properly compensated for, however other wavelengths in the operating band are left with significant dispersion due to the effect of the dispersion slope. The dispersion slope of standard single mode fiber at 1550 nm is typically on the order of 0.06 ps/nm2/km.
In order to achieve the high performance required by today""s communication systems, with their demand for ever increasing bit rates, it is necessary to reduce the effect of chromatic dispersion and slope. Several possible solutions are known to the art, including both active and passive methods of compensating for chromatic dispersion. One typical passive method involves the use of dispersion compensating (DC) fibers. DC fiber has dispersion properties that compensate for the chromatic dispersion inherent in optical communication systems. DC fibers exist that are designed to operate on both the fundamental or lowest order mode (LP01) and on higher order modes. Fibers designed to operate on higher order modes require the use of a mode converter so as to convert the optical signal from the fundamental mode to a high order mode. One desired property of DC fiber is that its dispersion should be of opposite sign of the dispersion of the transmission fiber that it is connected to. A large absolute value of dispersion of opposite sign reduces the length of fiber required to compensate for a large length of transmission fiber. Another desired property of a DC fiber is low optical signal attenuation. Ideally such a DC fiber should compensate for both chromatic dispersion and dispersion slope, and would be operative over the entire transmission bandwidth. The optical transmission bandwidth typically utilized is known as the xe2x80x9cCxe2x80x9d band, and is conventionally thought of as from 1525 nm-1565 nm. Longer wavelengths are also coming into usage, and are known as the xe2x80x9cLxe2x80x9d band, consisting of the wavelengths from 1565 nm-1610 nm.
Typical dispersion compensating fibers are designed as single mode fibers which support only the fundamental or lowest order spatial mode (LP01) at typical operating wavelengths. Such fibers are typically characterized as having relatively low negative dispersion, high loss, small Aeff and a resultant low tolerance for high power and limited compensation of slope, and are designed to compensate for transmission fibers exhibiting positive dispersion and positive dispersion slope, i.e. the dispersion increasing with increasing wavelength and is above zero in the operative band. Higher order spatial modes are typically not supported (i.e. not guided) through the fiber.
Other transmission fibers have been designed which exhibit negative dispersion and positive slope over the transmission band. Such fibers are disclosed for example in U.S. Pat. No. 6,091,873 and are conventionally known as negative non-zero dispersion shifted fibers (negative NZDSF), or reverse dispersion fibers (RDF). These fibers exhibit zero dispersion at a wavelength above the xe2x80x9cCxe2x80x9d band, and typically exhibit positive dispersion slope. One type of RDF exhibits dispersion at 1550 nm of xe2x88x921.32 ps/nm/km, with a slope of 0.053 ps/nm2/km. No effective method exists in the prior art for compensation for the dispersion of long lengths of these fibers. Standard single mode fiber has positive dispersion which may be utilized to compensate for the dispersion of the RDF, however its low dispersion, on the order of 17 ps/nm/km at 1550 nm requires a long length of fiber to compensate for the dispersion, thus incurring unwanted losses. In addition, the slope of the single mode fiber is of the same sign as the RDF, and thus does not compensate at all for the slope. There is thus a need for a fiber with strongly positive dispersion. It is also desirable that the fiber have a negative slope so as to compensate as well for the dispersion slope.
The term xcex94 is often used by itself in fiber design to define the relative difference in the maximum refractive index in a doped area (nmax) and the index of refraction of the cladding nclad, and is usually described as a percentage and defined by the equation xcex94=(nmax2xe2x88x92nclad2)/2nmax2 xc3x97100. Undoped silica cladding has a typical refractive index of 1.444 at a wavelength of 1550 nm.
The radii of the regions of the core are defined in terms of the index of refraction. A particular region begins at the point where the refractive index characteristic of that region begins, and a particular region ends at the last point where the refractive index is characteristic of that particular region. In general, we will use the point of return to the refractive index of the cladding to define the border between two adjacent regions that cross the cladding index. Radius will have this definition unless otherwise noted in the text.
Accordingly, it is a principal object of the present invention to overcome the disadvantages of the prior art in compensating for RDF. This is provided in the present invention by providing an optical waveguide having positive dispersion when operated substantially in a single high order mode.
In accordance with a preferred embodiment of the present invention, there is provided an optical waveguide having a refractive index profile pre-selected to have positive dispersion for optical signals in a high order mode, with the positive dispersion being greater than 50 ps/nm/km at a given wavelength within the operative range. In one embodiment the optical waveguide is a few mode fiber. In another embodiment the positive dispersion is greater than 100 ps/nm/km. In another embodiment the optical waveguide also has positive dispersion slope. In another embodiment the optical waveguide has negative dispersion slope. In yet another embodiment the optical waveguide has a nominally zero dispersion slope.
In a preferred embodiment the high order mode is the LP02 mode. In another preferred embodiment the high order mode is the LP03 mode.
The present invention also relates to a method of providing positive dispersion comprising the steps of providing an optical waveguide having a refractive index profile pre-selected to generate positive waveguide dispersion when operating in substantially a single high order mode, and operating the optical waveguide in said single high order mode in an operative range, whereby the total dispersion of the waveguide, equal to the sum of the material dispersion and its waveguide dispersion is greater than 50 ps/nm/km at a given wavelength within the operative range.
The present invention also relates to apparatus for introducing positive dispersion to an optical signal comprising at least one mode transformer and an optical waveguide having a refractive index profile pre-selected to generate positive dispersion to the optical signal when operated substantially in a single high order mode, the optical waveguide being in optical communication with the output of the mode transformer; whereby the optical signal is output from the mode transformer in the high order mode, and the output of the mode transformer is an optical signal substantially in the single high order mode.
In an exemplary embodiment the mode transformer is a transverse mode transformer.
Additional features and advantages of the invention will become apparent from the following drawings and description.